In many parts of the world contamination of the electromagnetic response by effect of magnetic soil, produces signals that can be interpreted as a response from a wanted target. The problem is most often encountered by users of electromagnetic detectors in search for unexploded ordnance, mines and buried treasures.
It is known in electromagnetic surveying techniques to differentiate responses from different types of targets by processing data obtained with respect to time or displacement so as to obtain additional data which can be used to provide the desired differentiation.
For example, in U.S. Pat. No. 5,654,637 (McNeill), responses from receiver coils at different distances above terrain are scaled and summed to eliminate responses from a particular depth, such as a susceptible surface layer. This technique depends on wanted and unwanted responses occurring at different depths.